The Gene Encoding the T-box Factor Tbx2 Is a Target for the Microphthalmia-associated Transcription Factor in Melanocytes

Abstract

Commitment to the melanocyte lineage is characterized by the onset of microphthalmia-associated transcription factor (Mitf) expression. Mitf plays a fundamental role in melanocyte development, with mice lacking Mitf being entirely devoid of pigment cells. In the absence of functional Mitf protein, melanoblasts expressing Mitf mRNA disappear around 2 days after their first appearance either by apoptosis or by losing their identity and adopting an alternative cell fate. The role of Mitf must therefore be to regulate genes required for melanoblast survival, proliferation, or the maintenance of melanoblast identity. Yet to date, Mitf has been shown to regulate genes such as Tyrosinase, Tyrp-1, and Dct, which are required for pigmentation, a differentiationspecific process. Because expression of these genes cannot account for the complete absence of pigment cells in Mitf-negative mice, Mitf must regulate the expression of other as yet uncharacterized genes. Here we provide several lines of evidence to suggest that Mitf may regulate the expression of the Tbx2 transcription factor, a member of the T-box family of proteins implicated in the maintenance of cell identity. First, isolation and sequencing of the entire murine Tbx2 gene revealed that the Tbx2 promoter contains a full consensus Mitf recognition element; second, Mitf could bind the promoter in vitro and activate Tbx2 expression in vivo in an E boxdependent fashion; and third, Tbx2 is expressed in melanoma cell lines expressing Mitf, but not in a line in which Mitf expression was not detectable. Taken together, with the fact that Tbx2 is expressed in Mitfpositive melanoblasts and melanocytes, but not in Mitfnegative melanoblast precursor cells, the evidence suggests that the Tbx2 gene may represent one of the first known targets for Mitf that is not a gene involved directly in the manufacture of pigment.Understanding how specific cell lineages are established and maintained lies at the heart of developmental biology. The melanocyte lineage arises in the neural crest as nonpigmented precursor cells, termed melanoblasts, which then migrate to their final destinations in the epidermis and hair follicles, where they differentiate into mature pigment-producing melanocytes. Little is known of the precise program of events leading from a multipotent neural crest cell to a melanoblast, but the switch from a melanoblast precursor cell to a melanoblast is characterized by the onset of expression of the basic-helixloop-helix-leucine zipper (bHLH-LZ)

Genomic profiling of malignant melanoma using tiling-resolution arrayCGH

“…TBX2 is one of the known targets for MITF in melanocytes, and a strong candidate melanoma oncogene (Carreira et al, 2000). Here, we showed that TBX2 has an increased gene copy number in 43% of the cell lines.…”

UV-mediated Regulation of the Anti-senescence Factor Tbx2

“…The Tbx2 promoter has been shown to possess an E-box (47), and it would be interesting to see if it is capable of responding to USF-1, particularly in response to UV irradiation.…”

Dual roles of lineage restricted transcription factors

“…22 These studies demonstrated that MITF transcriptionally upregulates DICER during cAMP induced melanocyte differentiation, causing DICER-dependent processing of the pre-miRNA-17_92 cluster, thus targeting BIM, which results in enhanced melanocyte survival. 22 Interestingly, MITF-dependent expression of TBX2 and DICER appears to occur after commitment to the melanocyte lineage 15 in the case of TBX2 or upon cAMP induced differentiation 22 in the case of DICER. Although certain MITF target genes are ubiquitously expressed (i.e., not restricted to the melanocyte lineage), their expression in melanocytes is MITF-dependent MiT family are thought to be more ubiquitously expressed.…”

“…Of course additional technical differences may account for variations in experimental observations, such became one of the first MITF target genes to be identified that was not involved in pigmentation, but rather in cell survival. 15 Later, additional MITF target genes important for the proliferation and survival of melanocyte were described: the anti apoptotic factor BCL2, 16 cyclindependent kinase 2 (CDK2), 17 the cyclin dependent kinase inhibitors p21 Cip1 (CDKN1A) 18 and p16/INK4A, 19 diaphanous-related forming 1 (DIA1), 20 and the c-MET receptor tyrosine kinase. 21 A recent observation by our laboratory revealed that, upon melanocyte differentiation, MITF induces the transcription of DICER, a key RNase III endonuclease crucial to miRNA biogenesis.…”